The present invention relates to optical coupling devices, and more particularly to a diffractive optical coupler that provides enhanced coupling efficiency.
Optical systems in which a light source is coupled to a waveguide are common. For example, fiber optic telecommunication systems often require the coupling of lasers to single-mode fibers, and parallel optical data links, which often require coupling of lasers to waveguides such as multimode graded-index optical fibers. Maximization of the coupled optical power to the fiber is an important goal regardless of whether the waveguide is a single mode or multimode waveguide. However, coupling to multimode waveguides is more complicated as they exhibit modal noise and bandwidth characteristics that depend on the coupling mechanism.
In addition, safety considerations become important whenever an end user can be exposed to optical radiation. Eye safety is enhanced by tailoring the optical beam to have uniform intensity over a wide numerical aperture (NA), and also by reducing the required power in the beam as much as possible by minimizing coupling loss. In optical fiber systems where the optical emissions exceed established standard eye safety limits, eye protection often requires the use of open fiber control(OFC) circuitry that acts to turn off the lasers should the link be broken. This circuitry increases system cost.
Several implementations of laser-to-waveguide coupling are known to the art. These can be divided into the two basic groups, refractive and diffractive optics. Methods involving refractive optics usually incorporate a lens somewhere in the optical interface to gather the laser light and transform its NA such that the light at the input to the waveguide is captured efficiently. Unfortunately, refractive optical coupling schemes are very sensitive to any misalignment, as such misalignments are magnified by the lens. These schemes also do not perform any beam shaping, and hence do little to address eye safety.
Methods that utilize a diffractive optical element (DOE) for expanding, collimating, and/or focusing light are also known to the art. However, these schemes do not match the light to the waveguide input NA profile across the waveguide, and hence, do little to address the eye safety issues.
Broadly, it is the object of the present invention to provide an improved coupling element for coupling a light source to an optical fiber or the like.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.
The present invention is an apparatus for coupling light from a light source into an optical waveguide having an entrance aperture for receiving light to be transmitted by the waveguide. The entrance aperture has a numerical aperture that may vary over the aperture. The apparatus includes an optical element for conditioning the light from the source and a diffractive optical element. Conditioning elements that collimate the beam or generate diverging or converging beams may be utilized. The diffractive optical element generates a plurality of light spots from the light source. The light from each light spot enters the entrance aperture of the waveguide at a different point on the entrance aperture. Each of the light spots has a numerical aperture that is less than the numerical aperture of the entrance aperture at the point on the entrance aperture at which the light from that light spot enters the entrance aperture. The optical conditioning element may be part of the diffractive optical element or separate therefrom. The apparatus may be constructed from a single optical element through which the light passes by stamping or molding the diffractive optical element into a surface of the optical element.